The Regan Isoenzyme of alkaline phosphatase of human cancer tissue is a carcinoplacental antigen. Thus, it is indistinguishable from the placental isoenzyme with respect to inhibition by L-phenylalanine, heat stability, starch gel electrophoresis, neuraminidase cleavage and enzyme kinetics. Lines of identity were observed in the Ouchterlony double diffusion technique between antisera to Regan and placental isoenzymes and the corresponding antigens and the same loss of activity occurs as a function of antisera dilution in the antigen-antibody complexes obtained separately with both antisera and both antigens. These findings have relevance to the hypothesis of gene derepression, a concept which may explain the many "ectopic" manifestations of human cancer. Our experimental work is directed to studying the morphology and genesis of such carcinoplacental antigens by the use of biochemical enzymologic and electron microscopic techniques. The possibility that the Regan isoenzyme may be an enzyme component of virus in cancer cells is also to be investigated. Simultaneously, sensitive and specific techniques for quantitating Regan isoenzyme in serum have been devised and collaborative work with clinicians is underway to ascertain the clinical utility of these methods. It is our hope to be able to find additional carcinoplacental antigens with enzyme activity.